Air valves for breathing systems are well known to the art. Typical of breathing systems is apparatus for administering an anesthetic gas to a patient which are referred to here by way of illustration. Such apparatus is normally provided with an emergency air intake valve to provide the patient with air in the event that there is a failure in the supply of the anesthetic gas. Typically the emergency air intake valve takes the form of a simple flapper valve which opens to admit outside air in the event of a negative pressure inside the anesthetic device. Such valves are unreliable since they tend to permit leakage therethrough of the anesthetic gas since the force exerted by the anesthetic gas operates on an area closely equal to the sealing area and hence is sometimes insufficient to keep the flapper valve tightly closed. In addition, the straight flow of such flapper valves, which are normally oriented with their open end up, encourages the entry of dirt into the anesthetic system.
These problems are solved by the invention by providing for a substantially greater force exerted by the anesthetic gas to operate on the sealing area. Further, the valve of the invention is constructed so that when it provides for the flow of air downwardly into an anesthetic device, the initial flow from the exterior is upwardly through openings in the bottom of the valve thus preventing the inflow of dirt.